2019ApJ...879...17B

We present high-resolution ALMA observations of the C¹⁸O, ¹³CO, and ¹²CO J = 1-0 isotopologues in three nearby ultraluminous infrared galaxies (ULIRGS; Arp 220, IRAS 13120-5453, and IRAS 17208-0014) and one nearby post-merger galaxy (NGC 2623). In all four systems, we measure high ¹²CO/C¹⁸O and ¹²CO/¹³CO integrated line ratios while the ¹³CO/C¹⁸O ratio is observed to be extremely low in comparison to typical star-forming disks, supporting previous work. We investigate whether these unusual line ratios are due to dynamical effects, astrochemistry within the gas, or nucleosynthesis in stars. Assuming both lines are optically thin, low ¹³CO/C¹⁸O values suggest that C¹⁸O is more abundant than ¹³CO in the interstellar medium of these systems. A plausible explanation is that local ULIRGs and their progeny have an excess in massive star formation; in other words, they are producing a top-heavy stellar initial mass function.